This invention relates to a method of manufacturing a semiconductor device from a silicon substrate.
In the manner known in the art, such a semiconductor device has a principal surface and an opposite surface opposite to the principal surface and includes a circuit element formed in a particular area, namely, a circuit element forming area through the principal surface. For manufacturing the semiconductor device, it is necessary to reduce contaminants, such as heavy metal, which are existent in the silicon substrate. This is because the contaminants result in deterioration of characteristics of the circuit element when they are existent in the silicon substrate.
To remove the contaminants from the silicon substrate, use is generally made of a process such as intrinsic gettering process (hereinafter abbreviated to an IG process) or extrinsic gettering process (hereinafter abbreviated to an EG process).
In the IG process, the contaminants are trapped in oxygen which is precipitated inside the silicon substrate, so that the amount of the contaminants is reduced from the particular area of the silicon substrate.
In order to precipitate the oxygen inside the silicon substrate, a first heat treatment is at first carried out as regards the silicon substrate at a high temperature to reduce oxygen concentration in the particular area of the silicon substrate. The first heat treatment is followed by a second heat treatment which is carried out at a low temperature for precipitating nuclei. Next, the second heat treatment is followed by a third heat treatment which is carried out at another high temperature for growing each of the nuclei to precipitate the oxygen. The oxygen concentration of 1-2.times.10.sup.18 atoms/cm.sup.3 is required in order to effectively carry out the IG process.
Next, the EG process will be described in the following. The silicon substrate has an opposite surface opposite to the principal surface. In the EG process, crystal loss is introduced in the opposite surface to trap the contaminants therein.
Introduction of the crystal loss is carried out prior to or during formation of the circuit element. Various methods can be used therefor. In a back side damaging method, a physical damage is given to the opposite surface of the silicon substrate by the use of fine particles such as SiO sprayed onto the opposite surface of the silicon substrate or a laser beam irradiated onto the opposite surface to melt and solidify the opposite surface. In an ion implantation method, the opposite surface is subjected to ion implantation to damage a crystal lattice known in the art. Use may be made of another method where a dopant is excessively diffused in the opposite surface to thereby deform the crystal lattice.
When a heat treatment is carried out, a number of crystal defects are induced in the opposite surface of the silicon substrate whose crystal lattice has been damaged by any of the above,mentioned methods. As a result, the contaminants are trapped in these crystal defects.
To remove the contaminants, use may be made of another process which will later be described referring to the drawing.
In each of the conventional methods, removal of the contaminants is restricted to those trapped in the oxygen and can not be sufficient. In particular, in case of a structure including a portion preliminarily covered by an oxidation film relatively thick, it is difficult to remove the contaminants trapped in the oxidation film.
Most of the contaminants trapped in the oxidation film are those existent in the particular area of the silicon substrate. Accordingly, the contaminants present in a relatively deep area can not sufficiently be removed.
Furthermore, when the oxygen is present as a precipitated oxygen in the particular area of the silicon substrate, the contaminants are trapped in the precipitated oxygen. Accordingly, the contaminants still remain in the particular area even after the oxidation film is removed.